The present invention relates to semiconductor storage devices and electronic equipment. More particularly, the invention relates to semiconductor storage devices as well as electronic equipment using the semiconductor storage devices, which include nonvolatile memory cells such as flash memory cells having a function that allows information to be stored therein by utilizing changes in cell currents.
In recent years, nonvolatile semiconductor storage devices such as flash memories and ferroelectric memories have been widely used as semiconductor storage devices for data storage or code (program) storage in portable telephones, digital cameras or the like. It is also under consideration to mount nonvolatile memories on glass substrates such as in liquid crystal panels.
Such nonvolatile memory cells store information by utilizing current differences among storage states. However, the nonvolatile memory cells structurally have large variations in cell characteristics. Specifically, there are differences in write time among respective memory cells or large variations in erase cell current. In particular, memory cells formed on a glass substrate are large in characteristic variations, giving rise to a need for picking up memory cells of extremely poor characteristics as failed products. However, it becomes very inefficient to treat an integrated liquid crystal panel as a failed product as the result of failure generated in only memory sections.
Also, the respective memory cells are affected each other in different proportions by disturbance (disturbance due to accesses to other memory cells), deterioration of endurance (deterioration in rewrite characteristics of memory cells due to increases in the number of times of rewrite), respective retention characteristics (stored-information retaining characteristics due to temperature variations, time variations) and the like. This leads to occurrence of product failures after their coming onto the market.
As a conventional typical method for read operation, there has been a semiconductor storage device wherein a current value or an average current value of reference cells is used as a reference current value, and a cell current value of a memory cell to be read is compared with the reference current value so as to determine information (see JP 2004-273093 A, for example). Specifically, data “0” and data “1” are stored in two reference cells, respectively, and their average current value is used as a reference current value.
However, the conventional semiconductor storage device has not correctly read information stored in memory cells in cases where the cell current is largely varied or where the characteristics of the respective memory cells are changed after the shipping of the products.